Field
The present disclosure relates to a binder composition, an anode including the binder composition, and a lithium battery including the anode.
Description of the Related Technology
Lithium batteries are widely used in various applications due to their high voltage and high energy density. Devices such as electric vehicles (HEV, PHEV), and the like require operation at high temperatures, being able to charge or discharge a large amount of electricity, and having long-term usability. Thus, the lithium batteries used in electric vehicles are required to have high-discharge capacity and better lifetime characteristics.
Carbonaceous materials are porous and can remain stable with little volumetric change during the charging and discharging process. However, carbonaceous materials may lead to a low-battery capacity due to the porous structure of carbon. For example, graphite, which is an ultra-high crystalline material, has a theoretical capacity density of about 372 mAh/g in the form of LiC6,
In addition, metals that are alloyable with lithium may be used as an anode active material for achieving a higher electrical capacity, as compared to carbonaceous materials. Examples of metals that are alloyable with lithium include silicon (Si), tin (Sn), aluminum (Al), and the like. These metals alloyable with lithium tend to easily deteriorate and have relatively poor lifetime characteristics. For example, during the repeated charging and discharging operations, the Sn particles used in battery may undergo repeated aggregation and breakage and resulting in electric disconnections.
Polyimides and polyamideimides that are commonly used in the art have been tested as binders in lithium batteries to suppress expansion of electrodes. However, these binders may not be practically applicable in lithium polymer batteries that are manufactured using the winding and pressing processes to harden the electrodes, because the binders may cause cracks in the electrodes during the manufacture process.
There is an increasing use of a diene-based copolymer as a binder in an anode. The diene-binder copolymer binder has good flexibility but may have a weak strength when impregnated with an electrolyte. Accordingly, the diene-binder copolymer may not suppress the expansion of the electrode when a non-carbonaceous, high-capacity anode active material, such as a Si or Sn metal alloyable with lithium, is used.
The diene-based copolymer binder may be prepared by emulsion polymerization, suspension polymerization, or the like. For example, a method of preparing a diene-based copolymer binder via emulsion polymerization to have a two-phase (core/shell) particle structure, or a method of continuously changing the composition of the diene-based copolymer binder via power feed polymerization have been introduced. However, such a polymer binder with core/shell particles or prepared by the continuous changing of the polymer composition may not maintain a good balance between the battery flexibility and the battery strength when impregnated with electrolyte.
For example, a glass transition temperature (Tg) of the shell regions of the core/shell structured binder particles may be increased to obtain binder particles with hard surfaces. However, the binder particles with hard surfaces may have poor film formability and may reduce the strength of the electrode. On the other hand, the Tg of the shell regions of the core/shell structured binder particles may be decreased to obtain the binder particles with soft surfaces. However, the binder particles with soft surfaces may have weak strength when combined with electrolyte. A polymer binder obtained with continuous changing of the polymer composition via power feed polymerization may not maintain a balance between flexibility and strength when combined with electrolyte.
Therefore, there is a need for a binder that may overcome the drawbacks of the conventional technologies and that may absorb and/or suppress a volumetric change of a non-carbonaceous anode active material.